Once you complete the game, a very powerful yellow cow will appear in Caladoxia Ruins. Unlock the doors to its room and lure it to the L-shaped staircase where you leave that part of the ruins; it will not be able to attack you now. Then, climb halfway down and go into Battle mode, and Terry will automatically keep attacking it with any weapon. It will take a very long time to kill and his weapon rank for that weapon and strength will max out after a while.